Since its invention over one hundred years ago the telephone has become one of the major vehicles for communication in the modern world. From an early point in the telephone's development, it became apparent that any large organization such as a business requiring a large number of telephones could advantageously provide a central switchboard that could direct incoming calls to a proper telephone line within the business. This avoids the necessity for having an individual dedicated line from a telephone company central office going to each telephone station within the organization. Such an arrangement has commonly been known as a private branch exchange (PBX).
In more recent times, both the call distribution functions and the seizure of central office telephone lines for outgoing calls have become automated or semiautomated. Whether automated or not, it is necessary in a private branch exchange to interface a number of trunk lines from a central telephone company office to a number of telephone lines within the PBX.
More recently, a confluence of legal and technical developments has made it highly desirable to provide a simple, inexpensive, versatile piece of apparatus for interfacing central office trunk lines with private branch exchange lines. Legal developments in the United States have been characterized by tendency to limit telephone utility monopolies to central office equipment and lines emanating therefrom. Regulating bodies, principally the FCC, have concluded that so long as PBX equipment is adequately designed so as not to damage telephone company central office equipment there is no rationale for preventing competition in the private branch exchange market. This development has naturally spawned a great deal of technical development and a proliferation of different types of private branch exchange equipment.
Modern private branch exchange interfaces conventionally provide four terminal pairs on the PBX side of the equipment for each central office trunk. These pairs are used for the audio signal path (customer tip and customer ring); a pair used for signal detection of a ring signal on the central office trunk, a pair for providing a supervisory signal indicating seizure of the central office trunk, and a pair for acceptance of status of a hook switch associated with the PBX line currently connected to the interface.
As is known to those skilled in the art, different telephone companies' central offices will respond to different signals provided by a customer on the central office trunk in order to effect seizure of a central office trunk line or to trip the ring. On central offices with ground start trunks, an outgoing call from the PBX to the central office is signaled by grounding a particular lead of the central office trunk, conventionally the central office ring conductor.
Other types of telephone central offices have loop start trunks which will respond to seize the central office line when closure of a current path between central office tip and central office ring is effected.
It is therefore advantageous to provide an interface between a central office trunk line and a generalized set of private branch exchange terminal pairs which is inexpensive, reliable, may be used with loop start and ground start central office equipment, and which allows a designer of the PBX to be faced with a minimum number of specifications for the equipment to be connected to the PBX terminal pairs.
Heretofore, interface devices for interfacing central office trunks and private branch exchange lines have been complex, relatively expensive, and have required a large number of different parts in order to accommodate both ground start trunks and loop start trunks.